Supersuperabsorbent polymers

ABSTRACT

The invention describes supersuperabsorbent polymers composed of surface-post crosslinked polycarboxypolysaccharides having excellent age-stable absorption properties, even under load, high attrition resistance and biodegradability and their use for absorbing water, aqueous or serous fluids and also blood. Also disclosed is a method of making which is impervious to changes in raw material quality and which provides consistent product quality.

This application is a continuation of International Application No.PCT/EP02/05799, internationally filed May 27, 2002.

FIELD OF THE INVENTION

The invention relates to superabsorbent polymers based on surfacemodified polycarboxypolysaccharides. The superabsorbent polymersaccording to the present invention possess a high absorption capacityand rate, even under pressure, for water and aqueous solutions; have nogel-blocking tendency; and are mechanically robust. The superabsorptionpolymers are age stable, toxicologically safe and biodegradable. Thepresent invention further relates to a simple process for preparing thesupersuperabsorbent polymers and their use in hygiene articles,food-packaging materials, culturing vessels, for soil improvement and ascable sheathing.

BACKGROUND OF THE INVENTION

Most of the superabsorption polymers used today for their ability totake up large amounts of liquid (water, urine) in a short time areprimarily lightly crosslinked synthetic polymers. They include, forexample, polymers and copolymers based on acrylic acid or acrylamide,which are not based on renewable raw materials and which areinsufficiently biodegradable, if at all.

Supersuperabsorbent polymers were initially developed with the focussolely on a very high swellability on contact with liquid, known asabsorption or free swelling capacity (FSC), but it was subsequentlydetermined that it is not just the amount of liquid which is absorbedthat is important but also the gel strength. Absorption capacity on onehand and gel strength of a crosslinked polymer on the other hand,however, are contrary properties, as is already disclosed in U.S. Pat.No. 3,247,171 and U.S. Pat. No. Re 32,649. Superabsorbent polymershaving a particularly high absorption capacity have little strength inthe swollen gel state, so that a confining pressure, for examplepressure due to the body of the wearer of a hygiene article, will causethe gel to deform and block further liquid distribution and absorption.According to U.S. Pat. No. Re 32,649, a balance should therefore besought between the absorption capacity and the gel strength in orderthat, when such supersuperabsorbent polymers are used in a diaperstructure, they ensure liquid absorption, liquid transport, diaperdryness and skin dryness.

A factor in this connection is not just that the freely swollensuperabsorbent polymer be able to retain the absorbed liquid under asubsequent application of a pressure, but also that the superabsorbentpolymer be capable of absorbing liquids even against a simultaneously(i.e. during the liquid absorption process) exerted pressure of the kindencountered in practice when an infant or adult sits or lies on asanitary article or when shearing forces are developed, for example as aresult of motion of the legs. This specific absorption characteristic isreferred to in Edana method 442.1-99 as “Absorbency Against Pressure” orAAP for short. The AAP value reported for a supersuperabsorbent polymeris based on the pressure employed, for example, 21 g/cm² at 0.3 psi and50 g/cm² at 0.7 psi. Also, the AAP may be based on the ratio chosen forthe measurement of the supersuperabsorbent polymer weight to area, forexample 0.032 g per cm², and also by the particle size distribution of agranular supersuperabsorbent polymer.

Patents EP 0 538 904 B1 and U.S. Pat. No. 5,247,072 disclosesupersuperabsorbent polymers based on carboxyalkyl polysaccharides. Toturn the carboxyalkyl polysaccharide into a supersuperabsorbent polymer,the carboxyalkyl polysaccharide is dissolved in water, isolated bydrying or precipitation and subsequently thermally crosslinked viainternal ester bridges formed by the reaction of the hydroxyl groups ofthe polysaccharide skeleton with the acidic carboxyl groups. Since thiscrosslinking reaction is very sensitive to small changes in the pH,temperature or reaction time, the superabsorbent polymers obtained havefluctuating absorption properties. The materials are notable for a highabsorbency under load value which, however, deteriorates to a fractionof the initial value after ageing for a few weeks.

U.S. Pat. No. 5,550,189 discloses superabsorbent polymers based oncarboxyalkyl polysaccharides that possess improved ageing stabilityowing to the addition of at least two-functional crosslinkers such asfor example aluminum salts or citric acid. The superabsorbent polymersare prepared from a conjoint homogeneous aqueous solution ofcarboxyalkyl polysaccharide and crosslinkers, in which solution thecomponents are present in low concentration and from which they areconjointly isolated and then thermally crosslinked. The synthesis ofthese superabsorbent polymers is very energy and time intensive, sincethe aqueous solutions are very weak. The improved ageing stability as itis reported in the majority of the illustrative embodiments does notmeet actual service requirements.

EP 855 405 A1 addresses the poor ageing stability of the absorptioncapacity of swellable starch maleates and proposes by way of solution tothis problem adding mercapto compounds to the double bond of the maleicacid substituent. The absorption performance of the product, especiallyunder a confining pressure, is very poor.

U.S. Pat. No. 4,952,550 describes a method of making a superabsorbentpolymer based on carboxymethylcellulose by treating thecarboxymethylcellulose in water or organic solvents with polyvalentmetal salts and a hydrophobicity agent. There is no thermal crosslinkingstep. According to the disclosure, the gel blocking of thesesuperabsorbent polymers is reduced by the hydrophobicity agent.

The raw materials for preparing polysaccharide based supersuperabsorbentpolymers are frequently soluble in water and have to be converted into awater-insoluble form for use as superabsorbent polymers for hygieneapplications. Numerous existing processes involve a homogeneouscrosslinking for the absorbent material in order that the watersolubility of the absorbent may be reduced. This frequently has thedisadvantage that such homogeneously crosslinked superabsorbent polymersno longer have the desired absorption capacity for liquids, since theswellability is excessively constrained by the crosslinking of thepolymer chains.

Furthermore, homogeneous crosslinking compromises the biodegradabilityof the superabsorbent polymer, since the constrained swelling reducesthe access for micro-organisms. In addition, the additionally introducedsubstituents inhibit enzymatic degradation [Mehltretter et al., Journalof the American Oil Chemists Society, 47 (1970) pages 522-524]. Attemptsto ameliorate these disadvantageous properties have led to varioussurface treatment proposals.

U.S. Pat. No. 5,811,531 discloses the preparation of a superabsorbentpolymer on the basis of polysaccharides, such as xanthan, which containuronic acid groups by reacting the polysaccharides at the surface withat least two-functional organic crosslinkers. According to thedisclosure, the products possess better free-swell absorbing abilityagainst salt solutions than carboxyalkylated polysaccharides where thecarboxyl groups are not attached directly to the saccharide units butvia alkyl groups.

U.S. Pat. No. 5,470,964 discloses a process for preparing asuperabsorbent polymer providing improved absorbency under load that isbased on polysaccharides containing acid groups and is surfacecrosslinked by polyvalent metal ions. The disadvantages of this processare that the improved absorbency under load is achieved by thecrosslinking of a relatively thick surface layer and that, according tothe disclosure, this is only possible through prior incipient swellingof the polysaccharide with a large amount of solvent. The incipientlyswollen state then allows sufficiently deep penetration of thepolyvalent metal ions into the surface. To achieve this, thepolysaccharide is introduced into an excess of the aqueous metal saltsolution such that the weight ratio of polysaccharide to water is from1:2 to 1:40. The thick crosslinked surface layer does provide goodabsorbency under load values, but the free swell capacity and also theretention capacity of the absorbent are disadvantageously reduced as aresult. The process described has the further disadvantage that thepolysaccharide portion added last to the crosslinker solution in thecourse of the manufacturing operation has less time to swell andencounters a lower crosslinker concentration, resulting in aninhomogeneous distribution of the crosslinker on the surface and hencefluctuations in the absorption properties.

U.S. Pat. No. 4,043,952 discloses the surface treatment ofwater-swellable anionic polyelectrolytes with polyvalent metal ions in adispersing medium in which the polymer is insoluble to improve thedispersibility of the water-absorbent products.

The broad object underlying the invention is to overcome thedisadvantages arising from the state of the art.

It is an object of the present invention to provide biodegradablesupersuperabsorbent polymers based on renewable raw materials that arefree of the defects described above. More particularly, thesuperabsorbent polymers shall have very long term storage stability withvery substantial retention of the absorption properties. The absorbentparticles shall also possess high mechanical robustness in order thatthe formation of fines in the course of processing operations such as,for example, screening or conveying may be avoided. Furthermore, withregard to the absorption performance, the superabsorbent polymers shallnot gel-block and shall possess not only a high absorption and retentioncapacity but also a high absorbency against pressure with regard towater and aqueous solutions. Moreover, for an effective absorption andin-use performance, the superabsorbent polymers shall have anoverwhelmingly insoluble character even in an excess of aqueoussolution.

It is a further object of the present invention to provide a process forpreparing such supersuperabsorbent polymers which is simple, economicaland safe to carry out, which provides consistent product quality andwhich utilizes little solvent and ideally no organic solvent. Moreover,the processes shall not require toxicologically suspect substances tocarry out.

A further object according to the invention consists in improving thebiodegradability of hygiene articles such as sanitary napkins, wounddressings, incontinence articles and diapers.

SUMMARY OF THE INVENTION

These objects are achieved by a post crosslinked a superabsorbentpolymer obtained by surface crosslinking at least one partly neutralizedcarboxyl-containing polysaccharide, characterized in that thepolycarboxypolysaccharide is aqueous preswollen in uncrosslinked formand dried before the surface crosslinking.

The present invention is directed to a superabsorbent polymer comprisingat least one partially neutralized, uncrosslinked, carboxyl-containingpolysaccharide that is preswelled and subsequently dried, wherein thedried polycarboxypolysaccharide is surface-post crosslinked by means ofa surface crosslinker.

In another embodiment, the present invention is directed to asuperabsorbent polymer comprising at least one partially neutralized,uncrosslinked, carboxyl-containing polysaccharide that is preswelled anddried and surface-post crosslinking the dried polycarboxypolysaccharideby means of a surface crosslinker, wherein the polycarboxypolysaccharideincludes one or more water-soluble additives from the group consistingof bases, salts and blowing agents or one or more anti-blockingadditives from the group consisting of natural fiber materials,synthetic fiber materials, silica gels, synthetic silicas andwater-insoluble mineral salts or any combination of at least two therefrom.

In another embodiment, the present invention is directed to a processfor preparing a superabsorbent polymers comprising the steps of a)forming a hydrogel by mixing an uncrosslinked polycarboxypolysaccharidewith water; b) the hydrogel is mechanically comminuted and dried; c) thedried hydrogel is comminuted and classified to form a polymer particles;and d) the superabsorbent polymer particles are coated with a solutionof a crosslinker and subjected to a surface post crosslinking.

The present is further directed to absorbent hygiene articles thatinclude the superabsorbent polymers of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a superabsorbent polymer comprisingat least one partially neutralized, uncrosslinked, carboxyl-containingpolysaccharide that is preswelled and subsequently dried, wherein thedried polycarboxypolysaccharide is surface-post crosslinked by means ofa surface crosslinker. One polysaccharide component used is apolycarboxypolysaccharide. Polycarboxypolysaccharides are either derivedfrom polysaccharides which contain no carboxyl groups and are providedwith carboxyl groups by subsequent modification or inherently alreadycontain carboxyl groups and may optionally be provided with furthercarboxyl groups by subsequent modification. The first group ofpolysaccharides includes for example starch, amylose, amylopectin,cellulose and polygalactomannans such as guar and carob bean flour whilethe second group includes for example xanthan, alginates, and gumarabic.

The carboxyl groups, as mentioned, are either present inherently fromthe given molecular construction, for example due to uronic acid unitsin the polysaccharide molecule, or are introduced by subsequentmodification with carboxyl-containing reagents or created by oxidationreactions. Of the polycarboxypolysaccharides where the carboxyl groupsare introduced by subsequent modification, preference is given to thecarboxyalkyl derivatives and especially to the carboxymethylderivatives. Of the polycarboxypolysaccharides where the carboxyl groupsare created by oxidation of the polysaccharide molecule, preference isgiven especially to oxidized starches and derivatives thereof.

The polycarboxypolysaccharides to be used according to the presentinvention are soluble or swellable in water and are used innon-crosslinked form. The polycarboxypoly-saccharides to be usedaccording to the invention, as well as containing carboxyl groups, maybe modified with further groups, especially with groups which improvethe solubility in water, for example hydroxyalkyl and especiallyhydroxyethyl groups and also phosphate groups. Preferredpolycarboxypolysaccharides are carboxymethylguar, carboxylatedhydroxyethyl or hydroxypropylcellulose, carboxymethylcellulose andcarboxymethylstarch, oxidized starch, carboxylated phosphatestarch,xanthan and mixtures thereof.

Polycarboxypolysaccharide derivatives having low and high degrees ofcarboxyl substitution are useful in the present invention. In apreferred embodiment, they have an average degree of carboxylsubstitution in the range from about 0.3 to about 1.5 and preferablypolycarboxypolysaccharide derivatives having a degree of substitution inthe range from about 0.4 to about 1.2.

The preferred water-soluble polycarboxypolysaccharide derivatives have ahigh average molecular weight for the molecular weight distributiondictated by the natural polymer construction and hence they also have ahigh solution viscosity in dilute aqueous solution like for examplecarboxymethylcellulose prepared from cotton linters. In the case ofcarboxymethylcellulose, useful derivatives have a 1% aqueous solutionviscosity of more than about 2000 mPas. Preference is given to usingcarboxymethylcellulose having a 1% aqueous solution viscosity of morethan about 5000 mPas and more preferably of more than about 7000 mPas.

Their method of making polycarboxypolysaccharides is such thatpolycarboxypolysaccharides may include variable amounts of salt as asecondary constituent. Typical salt levels of carboxymethylcelluloses infood grades are of the order of 0.5% by weight, while typical saltlevels of carboxymethylcelluloses in the case of technical grades rangefrom about 2% by weight up to 25 to 50% by weight for products used asprotective colloids. Although the superabsorbent polymers according tothe invention are very tolerant to a salt burden, thepolycarboxypolysaccharides to be used should not include more than about15% by weight, preferably not more than about 5% by weight and morepreferably not more than about 2% by weight of salt.

The superabsorbent polymers may be modified by addition of carboxyl-freepolysaccharides. Preference is given to using swelling polysaccharides,for example polygalactomannans or hydroxyalkylcelluloses. The amounts ofcarboxyl-free polysaccharides used for modifying purposes are determinedby the required performance profile and preference is given to usingabout 20% by weight, preferably about 10% by weight and more preferablyabout 5% by weight, based on the polycarboxypolysaccharide.

The carboxyl groups of the polycarboxypolysaccharides are at least about80%, preferably at least about 90% and most preferably 100% neutralized.Useful neutralizing agents are alkali metal hydroxides such as sodiumhydroxide, potassium hydroxide, sodium carbonate, potassium carbonate,sodium bicarbonate and potassium bicarbonate, ammonium hydroxide andamines.

The physical form of the polysaccharide derivatives used is immaterialto the properties of the superabsorbent polymers according to theinvention. The polysaccharide derivatives may therefore be used forexample in the form of powders, micro powders, granules, fibers, flakes,beads or compacts, in which case the use of pulverulent materials havinga particle size in the range from 1 to 2000 μm is preferable forsimplicity of metering and conveying.

The polycarboxypolysaccharide may be preswollen in an aqueous phaseincluding, based on the polycarboxypolysaccharide, about 0.01 to about20% by weight and preferably about 0.1 to about 10% by weight ofwater-soluble additives and about 0.01 to about 20% by weight andpreferably about 0.1 to about 10% by weight of antiblocking additive toimprove the processibility of the hydrogel being formed and to remain inthe product at least to some extent after drying.

Water-soluble additives for the purposes of the invention are selectedfrom the group consisting of bases, salts and blowing agents. Blowingagents are selected from inorganic or organic compounds which release agas under the influence of catalysts or heat, for example, from azo anddiazo compounds, carbonate salts, ammonium salts or urea.

Useful additives further include pH regulators such as for examplealkali metal hydroxides, ammonia, basic salts such as, for example,alkali metal carbonates or alkali metal acetates. Useful additivesfurther include neutral salts, for example, alkali metal or alkalineearth metal sulphates or chlorides, to regulate respectively the ionicstrength of the solution and the salt content of the pulverulentsuperabsorbent resin.

The aqueous hydrogel may further have added to it water-miscible organicsolvents, preferably water-miscible organic solvents which boil belowabout 100° C. These volatile organic solvents very substantially escapeagain from the hydrogel in the course of the subsequent drying step.These solvents are then completely volatilized in the course of thesubsequent surface post crosslinking.

Antiblocking additives to further reduce the gel-blocking tendency ofthe pulverulent absorbent resin include, for example, native orsynthetic fiber materials or other materials having a large surfacearea, for example, from the group consisting of silica gels, syntheticsilicas and water-insoluble mineral salts.

The superabsorbent polymers according to the invention are surface postcrosslinked. Following thermal drying, comminution and classification ofthe hydrogel, this crosslinking of the surface of thepolycarboxypolysaccharide powder is affected with covalent and/or ioniccrosslinkers which react with surface moieties, preferably carboxyl,carboxylate or hydroxyl groups, preferably by heating. Surfacecrosslinkers are used in an amount of about 0.01 to about 25% by weightand preferably about 0.1 to about 20% by weight based on thepolysaccharide.

Covalent surface post crosslinking agents, which may be used alone or incombination with ionic crosslinkers, include crosslinkers which reactwith the functional groups on the polycarboxypolysaccharide to formcovalent bonds. A preferred embodiment comprises using crosslinkerscapable of reacting with the hydroxyl groups of the absorbent resin, forexample, acid-functional substances.

Acid-functional substances include low molecular weight polycarboxylicacids and derivatives thereof, for example, malonic acid, maleic acid,maleic anhydride, tartaric acid and polymeric polycarboxylic acids, forexample, based on (meth)acrylic acid and or maleic acid. Preference isgiven to the use of citric acid, butanetetracarboxylic acid andpolyacrylic acid and particular preference is given to the use of citricacid. Citric acid is preferably used in an amount of about 0.2 to about8% by weight and more preferably about 0.3 to about 6% by weight basedon the polycarboxypolysaccharide. The polycarboxylic acids can also beused in partially neutralized form, for example, due to partialneutralization with alkali metal hydroxides or amine bases.

Ionic post crosslinking agents, which may be used alone or incombination with covalent post crosslinking agents, include salts of atleast divalent metal cations, for example, alkaline earth metal ionssuch as Mg²⁺, Ca²⁺ and also Al³⁺, Ti⁴⁺, Fe²⁺/Fe³⁺, Zn²⁺ or Zr⁴⁺, ofwhich Al³⁺, Ti⁴⁺ and Zr⁴⁺ are preferred and Al³⁺ is particularlypreferred. Aluminum salts are preferably used in an amount of about 0.2to about 1.0% by weight and preferably about 0.25 to about 0.85% byweight based on the polycarboxypolysaccharide.

The salts of the metal cations can be used not only alone but also mixedwith each other. The metal cations in the form of their salts possesssufficient solubility in the solvent used, and particular preference isgiven to metal salts with weakly complexing anions such as, for example,chloride, nitrate, sulphate and acetate.

Post crosslinking agents further include post crosslinking agentscapable of entering both covalent and ionic linkages, for example, di-and polyamines which can function not only as covalent crosslinkers, viaamide groups, but also as ionic crosslinkers, via ammonium saltcomplexes. The covalent surface post crosslinking may optionally beincreased by means of catalysts. Preferred catalysts are compounds whichcatalyze the esterification reaction between a carboxyl group and ahydroxyl group, for example, hypophosphites, acetylacetonates, mineralacids, for example, sulphuric acid, and Lewis acids. Preference is givento using sulphuric acid and hypophosphite. The weight ratio of surfacepost crosslinker to crosslinking catalyst is about 1:0001 to about 1:1and preferably about 1:0.1 to about 2:1.

The solution whereby the surface post crosslinker is applied to thepolycarboxypolysaccharide may optionally include one or morewater-soluble additives to promote a homogeneous distribution of thecrosslinker solution on the surface of the absorbent. In a preferredembodiment, the solution will include up to about 40% by weight of theseadditives. Such additives, as well as water-miscible organic solventssuch as, for example, ethanol, propanol, 2-propanol, acetone, glycerol,tetrahydrofuran and dioxane, also include water-soluble organic solidssuch as, for example, polyalkylene glycols, polyvinyl alcohols andpolyacrylic acids. Preference among organic solids is given to the useof polyethylene glycol. The preferred molecular weight range of thepolyethylene glycol is not less than 1000 and especially not less than1500.

In a preferred embodiment, the metal salts of divalent or higher cationsfunction both as ionic surface crosslinkers and as additives for ahomogeneous distribution of the crosslinker solution on the surface.

The particulate superabsorbent polymers according to the inventionexhibit very good retention and absorption ability and a significantlyimproved absorbency for water and aqueous fluids against an externalpressure in combination with an excellent ageing stability. The ageingstability shows itself in the fact that the absorbency against pressure(AAP_(0.7)) value after ageing for 200 days under standard conditions isat least 80% of the initial absorbency against pressure (AAP_(0.7))value.

The absorption properties of the superabsorbent polymers according tothe present invention show themselves in the fact that they can be madeto have a retention of not less than about 15 g/g coupled with anabsorbency against pressure (AAP_(0.7)) value of at least about 11 g/gand preferably of least about 15 g/g and preferably to have a retentionof not less than about 20 g/g coupled with an absorbency againstpressure (AAP_(0.7)) value of at least about 11 g/g and preferably of atleast about 15 g/g and in another embodiment to have a retention of notless than about 25 g/g coupled with an absorbency against pressure(AAP_(0.7)) value of at least about 11 g/g and preferably of at leastabout 15 g/g.

The bulk density of the particulate absorbent resins according to theinvention varies within the industrially customary range and is usuallybelow about 1000 g/dm³. In a preferred embodiment, the product has abulk density of less than about 800 g/dm³ and more preferably of lessthan about 650 g/dm³.

Another feature is the attrition stability of the superabsorbentpolymers according to the invention. Ball milling for 6 minutes (see“Mechanical Stability” test method) produces less than 5% of fines ofbelow 150 μm. This high attrition stability provides substantiallydustless processing of the superabsorbent polymers, for example, indiaper manufacturing equipment in which the superabsorbent polymers areexposed to mechanical stress in the course of conveyance.

Another feature is the biodegradability of the superabsorbent polymersunder composting conditions in that degradation to water and carbondioxide is at least 40% after 90 days and continues thereafter.

The surface post crosslinking according to the invention, incontradistinction to prior art products, is concentrated on a slightouter layer of extreme stability. This is determined by measuring thesurface crosslinking index (SCI), which is the difference between thecrosslinker concentrations in the attrited fines and the nonattritedabsorbent. The higher the SCI index, the greater the amount ofcrosslinker removed with the fines of the superabsorbent polymer, i.e.the greater the concentration in which the crosslinker is present on theouter layer of the superabsorbent polymer. Superabsorbent polymersaccording to the invention preferably have an SCI index of greater than40. When superabsorbent polymers have lower SCI values, the surfacecrosslinker has penetrated more deeply into the polymer particle,reducing the absorption properties.

The invention further provides a process for preparing the mechanicallystable, surface post crosslinked superabsorbent polymer particles havingsignificantly improved absorption properties coupled with consistentproduct quality by crosslinking the surface of apolycarboxypolysaccharide with a surface crosslinker, characterized inthat a hydrogel is formed from an uncrosslinkedpolycarboxypolysaccharide with water, mechanically comminuted and dried.The dried hydrogel is comminuted and classified to form a superabsorbentpolymer and in that the particles of the superabsorbent polymer arecoated with a solution of a crosslinker and subsequently subjected to asurface post crosslinking step.

The process according to the invention affords particulate absorbentresins having very good retention and absorption ability and asignificantly improved absorbency for water and aqueous fluids againstan external pressure in combination with an excellent ageing stabilityand also a distinctly reduced solubility in aqueous solutions.

The process according to the invention completely yields age stablesuperabsorbent polymer, which retain their very good absorptionproperties even when stored for a prolonged period, yet are continuouslybiodegraded under composting conditions.

The first step of the process according to the present inventionconverts the polycarboxypolysaccharide derivative together with asolvent into a solid hydrogel, which optionally additionally includesfurther additives. The solvent used is particularly preferably water ora mixture of water with organic solvents such as, for example, ethanol,propanol, butanol, 2-propanol or acetone. In an embodiment, thepolycarboxypoly-saccharide is presuspended in a mixture of water andorganic solvent, if appropriate under elevated temperature, andconverted into the hydrogel after separation from the suspension.

The hydrogel is preferably prepared by mechanically mixing thepolycarboxypolysaccharide derivative with the solvent component in acontinuous or batch operation. Suitable mixing means are, for example,batch kneaders such as trough kneaders, internal mixers or continuouskneaders such as single screw mixers or mixers having two or morescrews.

To prepare the hydrogel, the level of polycarboxypolysaccharide in themixture of polycarboxypolysaccharide and water can vary within widelimits. In one embodiment of the process, the level ofpolycarboxypolysaccharide in the mixture of polycarboxypolysaccharideand water is in the range from about 5 to about 65% by weight and morepreferably from about 5 to about 55% by weight. To facilitate processingof the hydrogel, it can occasionally be necessary for thepolycarboxypolysaccharide content not to exceed about 45% by weight. Inanother embodiment, the solvent is added to the drypolycarboxypolysaccharide raw material in a continuous operation, forexample, in an extruder, and the process is operated in such a way thatthe solvent is present in deficiency.

It was found that the absorption properties of the superabsorbentpolymers according to the present invention are minimally affected bythe affectivity of the mixing or the homogeneity of the initiallyprepared hydrogel. The mixing of the individual components in acontinuous mixing reactor with increasing throughput, for example, leadsto less homogeneous hydrogels having increasing fractions of dry,nonswollen polymer fractions. It is believed that a subsequent swellingprocess takes place in the course of the further processing topulverulent absorbent resins, so that the eventual absorptionperformance obtained is identical to that of completely homogeneouslymixed gels.

The mixture of polycarboxypolysaccharide and water may according to thepresent invention additionally include up to about 30% by weight andpreferably up to about 20% by weight of one or more organic solventsmiscible with water and immiscible with the polycarboxypolysaccharide.

The ratio of solid components to solvent components can vary within widelimits and is chosen so that the resulting hydrogel has a firm andminimally tacky consistency. It is advantageous for the swollen gel,having been conveyed using a mincer or extruder, for example, and shapedusing a breaker plate, to be in the form of firm extrudates which haveno tendency to mutual adherence even in the course of prolonged storage.Gel consistency can be specifically adjusted via the weight fraction oforganic water-soluble solvent in the hydrogel. The lower theconcentration of the polycarboxypolysaccharide derivative in thehydrogel, the higher the weight fraction of the organic solvent has tobe in order that the preferred gel consistency may be obtained. When,for example, the polycarboxypolysaccharide derivative used is a highmolecular weight carboxymethylcellulose having a 1% aqueous solutionviscosity of more than 4000 mPas and the solvent used is pure water, thepreferred gel consistency is obtained at a polymer content of more than15% by weight based on the swollen gel. Reducing the polymer fractionwithin the gel to less than 15% by weight gives a soft and tacky gel,which does not have the preferred consistency. However, on replacing1-20% and preferably 5-15% by weight of the water solvent with anorganic water-miscible solvent such as, for example, 2-propanol, whichis a coagulant for carboxymethylcellulose and decreases the solubilityof the polymer in the solvent mixture, even hydrogels having a polymerfraction of less than 15% by weight will have the preferred gelconsistency. Reducing the polymer fraction to less than 10% by weightrequires that the fraction of the organic solvent be correspondinglyfurther increased to more than 15% by weight in order that the preferredgel consistency may be obtained.

The presence of an organic water-soluble solvent in the swollen gel notonly has a positive effect on gel consistency, but also improves theabsorption properties of the pulverulent superabsorbent significantly.This effect becomes clearly apparent even at low levels of less than 5%by weight based on the gel and shows itself in the absorbent resinparticularly in a significantly higher absorption capacity for aqueousfluids against pressure.

The solvent or solvent mixture may further include 0.01-20% by weightand preferably 0.1-10% by weight, based on the solids content, of one ormore water-soluble additives from the group consisting of bases, saltsand blowing agents to improve the processibility of the swollen gel orthe absorption properties of the absorbent resin and also to suppressany crosslinking reaction during the drying operation. Preferredadditives are pH regulators such as, for example, alkali metalhydroxides, ammonia, basic salts such as, for example, alkali metalcarbonates or acetates. Preferred additives further include neutralsalts such as, for example, alkali metal or alkaline earth metalsulphates or chlorides for regulating the ionic strength of the solutionand the salt content of the pulverulent absorbent resin. Additionaladditives used are preferably compounds which release gases under theaction of catalysts or heat (blowing agents) and thus confer additionalporosity on the hydrogel or absorbent resin whereby the absorptionproperties of the absorbent resin are additionally improved. Examples ofblowing agents typically to be used are azo and diazo compounds,carbonate salts, ammonium salts and urea.

The hydrogel may further include 0.01-20% by weight preferably 0.1-10%by weight of one or more antiblocking additives to further reduce thegel-blocking characteristics of the pulverulent absorbent resin. Usefulantiblocking additives include, for example, native or synthetic fibermaterials or other materials having a large surface area, for example,from the group consisting of silica gels, synthetic silicas andsubstantially water-insoluble mineral salts.

In the next step of the process according to the invention, the hydrogelis comminuted and dried to low residual water content. The comminutingand drying step can immediately follow the preswelling step, but it isalso possible to store the hydrogels for a prolonged period, forexample, several weeks, prior to further processing without theproperties of the resulting superabsorbent polymers according to theinvention changing. Gel comminution particularly enlarges the ratio ofgel surface area to gel volume, as a result of which the subsequentdrying step requires substantially less energy input. The process of gelcomminution is not subject to any limitation. In a particularlypreferred embodiment, gel comminution is effected by pressing the gelthrough a breaker plate to form gel extrudates which may if appropriatebe divided into shorter gel extrudates by a cutting tool.

As regards drying the hydrogel particles, various processes are known.Possible processes include, for example, vaporizative drying,evaporative drying, irradiative drying (example: infrared drying), highfrequency drying (example: microwave drying), vacuum drying,freeze-drying or spray drying. The drying can accordingly be carriedout, for example, according to the thin film drying process, forexample, using a biaxial can dryer; according to the plate dryingprocess, whereby the hydrogel polymer particles are loaded on plates inmultiple layers into a drying chamber in which hot air circulates; bythe rotating drum process using can dryers; or by the conveyor beltprocess, herein below also referred to as belt drying. Belt drying,where foraminous trays of a circle conveyor are loaded in a tunnel withthe material to be dried and the material is dried by blowing hot airthrough the tray holes during the passage through the tunnel,constitutes the most economical drying process for water-swellablehydrophilic hydrogels and therefore is preferred.

The moisture content of the polymer powder formed by drying the hydrogelis advantageously not above 30% by weight, preferably not above 15% byweight and more preferably not above 10% by weight.

The addition polymer gel is dried at temperatures above about 70° C.,preferably above about 120° C. and more preferably above about 130°. Theparameters such as the polymer content of the hydrogel, the pH of thesolvent system, the method of mixing, the drying temperature and thedrying time are interdependent and are preferably attuned to each otherin such a way that no internal crosslinking of the hydrogel takes placeduring the drying step. If, for example, a solvent having a pH below 7is used to make the hydrogel, some of the carboxylate groups present inthe polysaccharide derivative are converted into the free acid form andare accordingly able, towards the end of the drying step in particular,to act as internal crosslinkers through an esterification with thehydroxyl groups. To control this fundamentally undesirable internalcrosslinking, the drying in these cases preferably takes place attemperatures in the range of 70-100° C. The pH is usually set to 6 orhigher. In a preferred embodiment of the invention, the hydrogel isprepared using a solvent having a pH of 7 or more and drying attemperatures of not less than 120° C., preferably from 130 to 160° C.

If the hydrogel is prepared in a continuous mixer, for example, anextruder, the precursor products obtained at a pH of not less than 7 andwhich as yet have not been surface post crosslinked may have highretention values of not less than 40 g/g, which turn out to be stable toheat treatment at 120° C. for 60 minutes and which differ only minimallyfrom products prepared at a higher pH. If, by contrast, the hydrogelsare prepared in a batch operation, the stability to heat treatmentincreases with increasing hydrogel pH. A preferred pH for hydrogelformation in a batch operation is pH 10 or higher.

It was found that, the particularly preferred drying temperatures ofabove about 130° C. provide superabsorbent polymers having asignificantly higher absorption and retention ability coupled withcomparable absorbency against an external pressure.

For the subsequent grinding of the dried hydrogel particles it isadvantageous to cool the dried material to temperatures of 70° C. orless, preferably 60° C. or less and more preferably 50° C. or less inthe last section of the preferred belt drying stage. The cooled driedhydrogel particles are initially prebroken, for example, by means of aknuckle-type crusher. The thus precomminuted hydrogel particles are thenground, preferably by means of a roll mill in order that the productionof fines may be minimized. In a particularly preferred embodiment, thegrinding is carried out in two stages, first via a coarse roll mill andthen via a fine roll mill, and the latter may in turn be carried out inone or two stages.

Screening is carried out subsequently to set the particle sizedistribution, which is generally between 10 and 3000 μm, preferablybetween 100 and 2000 μm and more preferably between 150 and 850 μm.Oversize particles may be resubmitted to grinding, while undersizeparticles may be recycled back into the forming operation.

The surface coating of the superabsorbent polymer with 0.01 to 25% byweight and preferably 0.1 to 20% by weight based on the addition polymerof a post crosslinker which is supplied in the form of a 0.01 to 80% byweight and preferably 0.1 to 60% by weight solution is carried out insuitable mixing assemblies. These are, for example, Paterson-Kellymixers, DRAIS turbulence mixers, Lödige mixers, Ruberg mixers, screwmixers, pan mixers, fluidized bed mixers or Schugi mixers. Theapplication of the crosslinker solution by spraying may be followed by aheat treatment step, preferably in a downstream dryer, at a temperaturebetween 40 and 250° C., preferably 60-200° C. and more preferably80-160° C. for a period of 5 minutes to 6 hours, preferably 10 minutesto 2 hours and more preferably 10 minutes to 1 hour to remove solventfractions. The optimum duration of the subsequent heating operation caneasily be determined for the individual crosslinker types in a fewexperiments. One limit for the duration is reached when the performanceprofile desired for the superabsorbent is destroyed again as aconsequence of heat damage. The thermal treatment can be carried out incustomary dryers or ovens; examples of suitable dryers and ovens arerotary tube ovens, fluidized bed dryers, pan dryers, paddle dryers andinfrared dryers.

It has been determined to be advantageous in some instances for theaqueous solution of the surface post crosslinker to be adjusted to atemperature of 15° C.-100° C. and preferably to 20° C.-60° C. beforeuse.

The time for covalent surface post crosslinking can be decreased by theuse of catalysts. Preferred catalysts are compounds which catalyze theesterification reaction between a carboxyl group and a hydroxyl group,for example hypophosphites, acetylacetonates, mineral acids, forexample, sulphuric acid, and Lewis acids. Preference is given to usingsulphuric acid and hypophosphite. The weight ratio of surface postcrosslinker to crosslinking catalyst is 1:0.001-1:1 and preferably1:0.1-2:1. In a preferred embodiment, the crosslinking catalysts aremixed into the solution of the surface post crosslinker.

The post crosslinking solution may optionally include up to 70% byweight of one or more additives. Additives are in particularwater-soluble compounds which promote a homogeneous distribution of thecrosslinker solution on the surface of the absorbent by slowing thepenetration of the solvent into the interior of the superabsorbentparticle and also reduce the solubility of the particle surface andhence the tendency of the moist superabsorbent particles to adhere toeach other. Preferred additives, as well as water-miscible organicsolvents such as, for example, ethanol, propanol, 2-propanol, acetone,glycerol, tetrahydrofuran and dioxane, also include water-solublehydrophilic organic solids, especially polymers such as, for example,polyalkylene glycols, polyvinyl alcohols and preferably polyethyleneglycols.

In a preferred embodiment, the metal salts of divalent or higher cationsfunction both as ionic surface crosslinkers and as additives for ahomogeneous distribution of the crosslinker solution on the surface.

The superabsorbent polymers according to the invention are notable forabsorption and retention ability for water, aqueous solutions and bodyfluids. At the same time, due to the controlled crosslinking of surface,the superabsorbent polymers possess an improved absorbency for aqueoussolutions against an external pressure. In addition, the superabsorbentpolymers according to the invention, which are based onpolycarboxypolysaccharide derivatives, are stable in storage, free ofresidual monomer fractions, only minimally soluble in aqueous fluids andbiodegradable.

The superabsorbent polymers according to the invention are very usefulas superabsorbent polymers in hygiene articles such as, for example,infant and adult diapers, wound contact materials, sanitary napkins,tampons and the like. The superabsorbent polymers are especiallysuitable for use in hygiene articles which are to be composted afteruse, since the polymers have proved biodegradable in composting tests inaccordance with ASTM method D 5338-92 of 15.12.1992; in accordance withthe CEN draft “Evaluation of the Ultimate Aerobic Biodegradability andDisintegration of Packaging Materials under Controlled CompostingConditions” of 6.5.1994; and in accordance with DIN 54900 Part 2 Method3 of January 1997.

Absorbent hygiene products typically possess a general constructioncomposed of a body facing liquid-pervious topsheet (1), aliquid-absorbent layer (2) and a substantially liquid-impervious bodyremote outer layer (3). Further structures may optionally findapplication in the absorbent core to rapidly acquire and distribute bodyfluid (4). These structures are frequently but not necessarily usedbetween the body facing liquid-pervious topsheet (1) and theliquid-absorbent layer (2).

The liquid-pervious topsheet (1) is typically composed of a fibrousnonwoven or some other porous structure. Useful materials for thistopsheet (1) include, for example, synthetic polymers such as polyvinylchloride, polyvinyl fluoride, polytetrafluoroethylene (PTFE), polyvinylalcohols and derivatives, polyacrylates, polyamides, polyesters,polyurethanes, polystyrene, polysiloxanes or polyolefins (e.g.polyethylene (PE) or polypropylene (PP)) and also natural fibermaterials and also any desired combinations of the aforementionedmaterials in the form of hybrid materials, composite materials orcopolymers.

The liquid-pervious topsheet (1) has a hydrophilic character. It mayalso constitute a combination of hydrophilic and hydrophobicconstituents. Preference is generally given to a hydrophilic finish forthe liquid-pervious topsheet (1) in order that rapid seepage of bodyfluid into the liquid-absorbent layer (2) may be ensured, but partiallyhydrophobicized topsheets (1) are used as well.

The liquid-absorbent layer (2) includes the superabsorbent powders orgranules according to the invention and further components composed, forexample, of fibrous materials, foam materials, film-forming materials orporous materials and also combinations of two or more thereof. Each ofthese materials can be of natural or synthetic origin and may have beenprepared by chemical or physical modification of natural materials. Thematerials can be hydrophilic or hydrophobic, in which case hydrophilicmaterials are preferred. This applies especially to those compositionswhich are to efficiently acquire secreted body fluids and transport themin the direction of regions of the absorbent core which are more remotefrom the point of ingress of the body fluid.

Useful hydrophilic fiber materials include, for example, cellulosicfibers, modified cellulosic fibers (for example stiffened cellulosicfibers), polyester fibers (for example, Dacron), hydrophilic nylon orelse hydrophilicized hydrophobic fibers, for example,surfactant-hydrophilicized polyolefins (PE, PP), polyesters,polyacrylates, polyamides, polystyrene, polyurethanes and others.

Preference is given to using cellulosic fibers and modified cellulosicfibers.

Combinations of cellulosic fibers and/or modified cellulosic fibers withsynthetic fibers such as, for example, PE/PP bicomponent fibers as used,for example, to thermobond airlaid materials or other materials arelikewise customary. The fiber materials can be present in various useforms, for example, as loose cellulosic fibers deposited or laid downfrom an air stream or from an aqueous phase, as a nonwoven or as atissue. Combinations of various use forms are possible.

The superabsorbent polymers according to the invention may optionallyinclude further pulverulent substances, for example, odor-bindingsubstances such as cyclodextrins, zeolites, inorganic or organic saltsand similar materials.

The liquid-absorbent layer (2) may be mechanically stabilized usingthermoplastic fibers (for example, bicomponent fibers composed ofpolyolefins), polyolefin granules, latex dispersions or hot meltadhesives. Optionally, one or more layers of tissue are used forstabilization.

The liquid-absorbent layer (2) can be a single layer or be composed of aplurality of layers. Preference is given to the use of structuresconstructed of hydrophilic fibers, preferably cellulosic fibers,optionally a structure to rapidly acquire and distribute body fluid (4)such as, for example, chemically stiffened (modified) cellulosic fibersor high loft webs composed of hydrophilic or hydrophilicized fibers andalso superabsorbent polymers.

The superabsorbent polymer according to the invention can behomogeneously distributed in the cellulosic fibers or stiffenedcellulosic fibers, it can form a layer between the cellulosic fibers orstiffened cellulosic fibers, or the concentration of the superabsorbentpolymer can have a gradient within the cellulosic fibers or stiffenedcellulosic fibers. The ratio of the total amount of superabsorbentpolymer and of the total amount of cellulosic fibers or stiffenedcellulosic fibers in the absorbent core can vary between 0:100 and70:30%, although one embodiment provides local concentrations of up to100% of superabsorbent, for example, in the case of gradiented orlayered incorporation. Such structures feature regions of highconcentrations of superabsorbent polymer, the fraction of superabsorbentbeing between 60 and 100% and most preferably between 90% and 100% incertain regions.

It is optionally to use at the same time two or more differentsuperabsorbent polymers which differ, for example, in the absorptionrate, the permeability, the storage capacity, the absorbency againstpressure, the particle size distribution or else the chemicalcomposition. The various superabsorbent polymers can be introduced intothe absorbent core after blending with each other or else can be placedin the absorbent pad with local differentiation. Such a differentiatedplacing can be effected in the direction of the thickness of theabsorbent core or in the direction of the length or in the direction ofthe width of the absorbent pad.

The liquid-absorbent layer (2) includes one or more of theabove-described cellulosic fiber or stiffened cellulosic fiber layerscontaining superabsorbent polymers. A preferred embodiment utilizesstructures composed of combinations of layers featuring homogeneoussuperabsorbent incorporation and additionally layered incorporation.

These aforementioned structures are optionally also supplemented byfurther layers of pure cellulosic fibers or stiffened cellulosic fiberson the body facing side and/or else the body remote side. Theabove-described structures can also repeat a number of times, in whichcase there may be a superposition of two or more identical layers orelse a superposition of two or more different structures. Thedifferences are in turn purely structural or else in the type ofmaterial used, for example, the use of superabsorbent polymers differingin terms of properties or else the use of different pulp varieties.Optionally, the entire absorbent pad or else individual layers of theliquid-absorbent layer (2) are separated from other components by layersof tissue or are in direct contact with other layers or components.

By way of example, the structure for rapid acquisition and distributionof body fluid (4) and the liquid-absorbent layer (2) can be separatedfrom each other by tissue or else be in direct contact with each other.If there is no separate structure to rapidly acquire and distribute bodyfluid (4) between the liquid-absorbent layer (2) and the body facingliquid-pervious topsheet (1), but the fluid distribution effect is to beachieved, for example, by the use of a specific body facingliquid-pervious topsheet (1), the liquid-absorbent layer (2) canoptionally likewise be separated from the body facing liquid-pervioustopsheet (1) by a tissue.

Instead of tissue it is optionally also possible for a nonwoven to beincorporated into the liquid-absorbent layer (2). Either componentbrings about the desired secondary effect of stabilizing andstrengthening the absorbent core in the moist state.

Fibrous layers which contain superabsorbent and distribute and storeliquid can be generated using a multiplicity of production processes. Aswell as the established conventional operations as generally subsumed bythose skilled in the art under drum forming using forming wheels,forming pockets and product forms and correspondingly adapted meteringmeans for the raw materials, customary methods for producing theabovementioned liquid stores include modern established processes suchas airlaid, with all forms of metering, fiber letdown and consolidationsuch as hydrogen bonding, thermal bonding, latex bonding and hybridbonding, wetlaid, carding, meltblown and spunblown operations andsimilar operations for producing superabsorbent-containing nonwovenssingly and combined with and among each other.

Further processes include the production of laminates in the widestsense and also of extruded and coextruded, wet-consolidated anddry-consolidated and also subsequently consolidated structures. Acombination of these processes with and among each other is likewisepossible.

A structure for rapid acquisition and distribution of body fluid (4) iscomposed, for example, of chemically stiffened (modified) cellulosicfibers or high loft webs composed of hydrophilic or hydrophilicizedfibers or a combination of both.

Chemically stiffened, modified cellulosic fibers can be produced, forexample, from cellulosic fibers which are reacted by means ofcrosslinkers such as, for example, C₂-C₈ dialdehydes, C₂-C₈monoaldehydes having an additional acid function or C₂-C₉ polycarboxylicacids in a chemical reaction. Specific examples are glutaraldehyde,glyoxal, glyoxalic acid or citric acid. Also known are cationicallymodified starch or polyamide-epichlorohydrin resins (i.e., KYMENE® 557H,Hercules Inc., Wilmington, Del.). The crosslinking provides andstabilizes a twisted, curled structure which has an advantageous effecton the rate of fluid acquisition.

The absorbent hygiene products can differ widely in basis weight andthickness and hence density. Typically the densities of the regions ofthe absorbent cores are between 0.08 and 0.25 g/cm³. The basis weightsare between 10 and 1000 g/m², although it is preferable to provide basisweights between 100 and 600 g/m². The density varies in general alongthe length of the absorbent core. This is a consequence of a controlledmetering of the cellulosic fiber or stiffened cellulosic fiber quantityor the quantity of the superabsorbent polymer, since these componentsare in preferred embodiments preferentially incorporated in the frontregion of the disposable absorbent article.

This controlled increase in the absorbent material in certain regions ofthe absorbent core can also be achieved, for example, by producing anappropriately sized airlaid or wetlaid sheet material composed ofhydrophilic cellulosic fibers, optionally of stiffened cellulosicfibers, optionally of synthetic fibers (e.g. polyolefins) and also ofsuperabsorbent polymers and subsequent back rolling or superposition.

The polymers according to the invention are also used in absorbentarticles which are suitable for a wide variety of uses, for example, bymixing with paper or fluff or synthetic fibers or by distributing thesuperabsorbent polymers between substrates of paper, fluff or nonwoventextiles or by processing into base materials to form a continuouslength. The polymers according to the invention are further usedwherever aqueous fluids have to be absorbed, for example, in cablesheaths, in food packaging, in the agricultural sector for plantcultivation and as water storage medium and also as a carrier for anactive component to be released to the environment in a controlledmanner.

The products according to the invention which have a good combination ofvery high absorption and retention values, excellent absorbency againstpressure and biodegradability can be prepared without the use oftoxicologically compromised substances. According to the invention, thepolymers can be produced on a large industrial scale according toexisting processes in a continuous or batch wise manner and withconsistent product quality.

The invention is further concerned with structures for absorbing bodyfluids, comprising a polymer according to the invention. Theseaforementioned structures are preferably absorbent bodies. In anotherembodiment of the construction it is a sanitary napkin, a diaper or anincontinence product, wherein diapers are particularly preferred.

Test Methods

Retention (TB)

The retention values were determined by performing a tea bag test. Thetest solution used was a 0.9% strength NaCl solution. 0.20 g of the testsubstance (screened off between 150 and 850 μm) were sealed into a teabag and immersed in the test solution for 30 minutes. The tea bag wassubsequently spun in a centrifuge, for example, a commercially availablelaundry spin dryer, at 1400 rpm for 3 minutes. The amount of liquidabsorbed was determined gravimetrically after subtraction of the blankvalue (weight of an empty tea bag after spinning) and converted to 1 gof test substance. The retention value corresponds to the amount ofliquid absorbed in grams per gram of test substance.

Absorbency Against a Pressure of 0.3 or 0.7 psi (AAP)

The ability to absorb a liquid against an external pressure (absorbencyagainst pressure, AAP) was determined as per Edana method No. 442.1-99.0.90 g of the test substance (screened off between 150 and 850 μm) wasweighed into a test cylinder having an internal diameter of 60.0 mm anda 400 mesh screen base (concentration: 0.032 g/cm²) and uniformlydistributed therein. Onto the test substance is placed a cylindricalweight (21 g/cm²=0.3 psi or 50 g/cm²=0.7 psi) having an outer diameterof 59.2 mm. Filter plates covered with a filter paper are placed in aplastic dish. The plastic dish is filled with 0.9% strength NaClsolution until the surface of the liquid is flush with the upper edge ofthe filter plates. The prepared measuring units are then placed on thefilter plates. After a swell time of 60 minutes the measuring units aretaken out and the weight is removed. The amount of liquid absorbed isdetermined gravimetrically and converted to 1 gram of test substance.

Extractables (EA)

Extractable fractions in the biodegradable superabsorbent resins weredetermined by GPC analysis under the following test conditions.

Column material: HEMA BIO 40, column length: 300 mm, column diameter: 8mm eluent: 0.9% NaCl solution, flow rate: 1.0 ml/minute, temperature:room temperature, injection volume: 50 μl, running time 15 minutes,calibrating substance: low viscosity carboxymethylcellulose (Finnfix®4000 G).

0.50 g of the test substance is admixed with 100 ml of 0.9% strengthNaCl solution and stirred for 16 hours. After filtration through a glassfilter crucible (pore size 1) the filtrate was diluted with the eluentin a ratio of 1:10. This dilution was injected and the area value of thepolymer peak determined. The soluble fraction of the test substance wascalculated by means of a calibration curve prepared with the lowviscosity carboxymethylcellulose under identical conditions.

Fluff-absorbent Combination Test (FACT)

2.0 g of cellulose fluff were weighed out on an analytical balance andformed into three fluff layers. 0.22 to 2 g (=10 to 50% by weight) ofsuperabsorbent polymer was uniformly sprinkled between the fluff layersso as to create a fluff/SAP/fluff/SAP/fluff sandwich. Thefluff-superabsorbent pad was placed in a test apparatus having a screenbase and weighted with a metal ring to stop the absorbent resin escapingfrom the test apparatus as it swells. The test specimen was loaded witha weight (21 g/cm² or 50 g/cm² corresponding to 0.3 psi and 0.7 psirespectively). The test specimen was then allowed to swell in 0.9%strength NaCl solution by capillary action while the absorption wasrecorded by electronic data processing. The test was deemed to haveended when less than 1 g of test liquid was absorbed in the course of 10minutes. For every measurement, the amount of liquid absorbed wasplotted against time in an absorption curve from which the followingparameters were determined:

-   a) maximally attained end value in grams: Abs_(max)-   b) time [min] at which the end value was attained: t_(max)-   c) time [min] at which x% of the final value was attained: t_(x%)    Airlaid Tests

An airlaid machine was used to fabricate composite materials composed ofone layer of tissue, a subsequent layer of a cellulose fluff-absorbentpowder mix and a further layer of tissue. Round specimens 6 cm indiameter were die cut from the composite and used for the subsequenttests.

Composite Retention

A tea bag test was carried out to determine the retention values of thecomposite. The test solution used was a 0.9% strength NaCl solution. Adie cut composite specimen was weighed, sealed into a tea bag andimmersed in the test solution for 30 minutes. The tea bag wassubsequently spun in a centrifuge, for example, a commercially availablelaundry spin dryer, at 1400 rpm for 3 minutes. The amount of liquidabsorbed was determined gravimetrically after subtraction of the blankvalue (weight of empty tea bag after spinning) and converted to 1 m² ofcomposite. The retention value corresponds to the amount of liquidabsorbed in grams per square meter of airlaid composite.

Absorbency of a composite against a pressure of 20 or 50 g/cm²(LAUL20/LAUL50). A die cut composite specimen was weighed into a testcylinder having an internal diameter of 60.0 mm and a 400-mesh screenbase. A cylindrical weight (20 g/cm² or 50 g/cm²) having an externaldiameter of 59.2 mm is placed onto the test substance. Filter plates areplaced into a plastic dish and covered with a filter paper. The plasticdish is filled with 0.9% strength NaCl solution until the surface of theliquid is level with the upper edge of the filter plates. The preparedmeasuring units are then placed on the filter plates. After a swell timeof 60 minutes the measuring units are taken out and the weight isremoved. The amount of liquid absorbed is determined gravimetrically andconverted to 1 square meter of airlaid composite.

Mechanical Stability

127 g of grinding media (cylindrical pieces of porcelain, U.S. Stoneware½″ O.D. * ½″) and 10 g of a pulverulent superabsorbent resin having aparticle size of 150 to 850 μm were weighed into a ball mill pot. Theball mill pot was sealed and rotated at 95 rpm on a roll mill for 6minutes. The mechanically stressed superabsorbent was taken from the potand analyzed with regard to particle size distribution.

Surface Crosslinking Index (SCI)

127 g of grinding media (cylindrical pieces of porcelain, U.S. Stoneware½″ O.D. * ½″) and 10 g of a surface-crosslinked superabsorbent resinhaving a particle size of 150 to 850 μm were weighed into a ball millpot. The ball mill pot was sealed and rotated at 95 rpm on a roll millfor 30 minutes. The mechanically stressed superabsorbent was taken fromthe pot and the particles having a particle size <150 μm were screenedout. The screened-off fines were digested with HNO₃ and H₂O₂ using amicrowave and subsequently hydrolyzed with water. The aluminum contentwas then determined photometrically via the yellowish red AlizarinS-aluminum complex. The SCI is calculated from the amount of Al³⁺ addedin the course of the surficial crosslinking, based on the superabsorbentresin (=C_(SAP)), and the Al³⁺ concentration of the fine particles foundafter mechanical exposure (=C_(F)) in accordance with the equationSCI=(C_(F)−C_(SAP))*100, where C_(F) and C_(SAP) are inserted in % Al³⁺.

EXAMPLES

All superabsorbent polymers according to the invention were, unlessotherwise stated, ground prior to surface coating and screened off to aparticle size of 150 to 850 μm. The moisture content of all pulverulentabsorbent resins was less than 10% by weight.

Example 1 Preparation of Precursor Hydrogel

In a make-up vessel, 100 g of carboxymethylcellulose (CMC) weresuspended in a mixture of 244 g of 2-propanol and 156 g of DM water andrefluxed for 1 hour. After the suspension had been cooled to roomtemperature, the carboxymethylcellulose was filtered off. A secondmake-up vessel was charged with 900 g of water which was adjusted to pH9 with NaOH. The filtered-off carboxymethylcellulose was introduced intothe second make-up vessel with vigorous stirring to form a firmhydrogel. After a swell time of 30 minutes, the swollen hydrogel was fedinto a meat mincer equipped with a mincer plate and comminuted. Thecomminuted hydrogel was dried at 80° C. in a circulating air cabinet for12 hours. The dried hydrogel was coarsely comminuted and ground using aRetsch mill. After the particle size fraction of 150 to 850 μm had beenscreened off, the retention value of the uncrosslinked precursor wasdetermined. Table I includes various precursor variations and theretention values thus determined for various commercially availablecarboxymethylcelluloses (CMCs):

TABLE 1 Precursor Variations of Example 1 Precursor Viscosity retentionNo CMC [mPas] D.S.^([a]) [g/g] 1.1 Finnfix ® 50,000^([b])  8,200 (1%)0.78 46.5 1.2 Cekol ® 50,000^([b])  8,400 (1%) 0.72 47.0 1.3 Cekol ®100,000^([b])  10,000 (1%) 0.76 32.2 1.4 Tylose ® CB30,000^([c]) >24,000 (2%) >0.85  45.3 1.5 Blanose ® 7HOF^([d])   2140(1%) 0.85 36.8 1.6 Walocel ® VP-C-2204^([e])   >7500 (1%) 0.65-0.95 44.7^([a])Degree of substitution as per manufacturer data, ^([b])Noviant,^([c])Clariant, ^([d])Aqualon, ^([e])Wolff-Walsrode

Example 2

A crosslinker solution was prepared from 1.29 g of citric acidmonohydrate, 61 g of 2-propanol and 39 g of DM water. 10 g of each ofthe pulverulent precursors prepared according to Example 1) were eachcoated with 4 g of this crosslinker solution (corresponding to a citricacid concentration of 0.47% based on CMC) and dried at 80° C. for 2hours. Surface crosslinking was completed by a subsequent annealing stepfor the stated period at 120° C. The annealing time was chosen so as toensure a balanced ratio of retention to absorbency against pressure. Thesuperabsorbent polymers thus prepared had the following characteristicdata as shown in Table 2:

TABLE 2 Example 2 Variations Precursor Annealing TB AAP_(0.3) AAP_(0.7)No of inv. ex. [min] [g/g] [g/g] [g/g] 2.1 1.1 30 24.0 21.6 14.4 2.2 1.250 21.0 20.5 16.1 2.3 1.3 30 19.4 20.9 16.8 2.4 1.4 30 20.4 21.8 17.2

Example 3

Various crosslinker solutions were prepared by adding acetone to anaqueous solution of aluminum sulphate 18-hydrate in DM water:

A: 13 g of Al₂(SO₄)₃*18 H₂O/100 g of DM water and 36.7 g of acetone

B: 18 g of Al₂(SO₄)₃*18 H₂O/100 g of DM water and 36.1 g of acetone

10 g of each of the precursors of Example 1 were each coated with 4 g ofeach crosslinker solution thus prepared by initially charging thepulverulent precursor and adding the crosslinker solution drop wise withstirring. The coated product was dried at 80° C. for 2 hours and thedried products were measured for retention and APP:

TABLE 3 Example 3 Variations Precursor Crosslinker TB AAP_(0.3)AAP_(0.7) No of inv. ex. solution % Al³⁺/CMC [g/g] [g/g] [g/g] 3.1 1.1 B0.38 24.1 18.9 13.6 3.2 1.2 A 0.28 22.6 20.5 16.1 3.3 1.3 B 0.38 18.317.9 14.8 3.4 1.6 B 0.38 22.8 18.6 14.1

Example 4

Carboxymethylcellulose (Finnfix® 50,000) was preswollen and dried, bothsteps being carried out as described in Example 1). 50 g of theuncrosslinked precursor thus prepared were weighed into a plastic cupand stirred using a household mixer. 3 g of a solution consisting of3.33 g of citric acid monohydrate and 17.5 g of polyethylene glycol (1500 g/mol) in 29.2 g of DM water were poured onto the pulverulentprecursor in the course of 10 seconds, followed by stirring for afurther 100 seconds. The coated absorbent was cured at 120° C. for 45minutes and showed the following characteristic data:

Example No. 4.1: TB = 22.2 g/g AAP_(0.7) = 13.3 g/g

Example 5

Example 4) was repeated, except that the 3.0 g of coating solution usedwas composed of 5.5 g of polyacrylic acid (M_(w) 1 500 g/mol) and 0.39 gof sodium hydroxide in 10.0 g of DM water:

Example No. 5.1: TB = 23.2 g/g AAP_(0.7) = 13.0 g/g

Example 6

Carboxymethylcellulose (Finnfix® 50,000) was mixed with the statedamounts of guar bean flour. The powder mixture was preswollen and dried,both steps being carried out as described in Example 1). 10 g of each ofthe dried pulverulent precursors are coated with 4 g of a solution of0.85 g of citric acid monohydrate in 99.15 g of 2-propanol and dried at80° C. for 2 hours. The crosslinking reaction was then completed at 120°C. for 30 minutes.

TABLE 4 Example 4 Variations % by weight of TB AAP_(0.3) AAP_(0.7) Noguar bean flour [g/g] [g/g] [g/g] 6.1 5 22.4 19.1 12.7 6.2 10 22.5 17.513.1 6.3 20 16.2 16.5 12.8

Example 7

In a make-up vessel, 600 g of carboxymethylcellulose (Cekol® 50,000,degree of neutralization 98.6%) were suspended in a mixture of 1,460 gof 2-propanol and 940 g of DM water and refluxed for 1 hour. Thesuspension was cooled down to room temperature and filtered. A secondmake-up vessel was charged with 5000 g of DM water which were thenadjusted to pH 9 with 2.5 g of 10% strength aqueous sodium hydroxidesolution. The filter cake was introduced into the second make-up vesselwith strong stirring and the hydrogel formed was comminuted after 1 hourin a meat mincer equipped with a mincer plate. The comminuted hydrogelwas divided into two halves and dried on wire mesh at differenttemperatures. The dried hydrogels were coarsely comminuted, ground witha Retsch mill and screened off to a particle size of 150 to 850 μm. Thescreened-off pulverulent superabsorbent polymers were measured forretention.

In a further run, the pH in the make-up vessel was adjusted to pH 11.3with 11.5 g of 10% strength aqueous sodium hydroxide solution. Theproducts resulting there from were coated with the crosslinking solutionbased on citric acid as per Example 2 and post crosslinked at 120° C.for 50 minutes.

TABLE 5 Example 7 Variations Post crosslinked product Precursor DryingTB TB AAP_(0.3) AAP_(0.7) No pH [° C./h] [g/g] [g/g] [g/g] [g/g] 7.1  980/12 45.2 7.2  9  150/2.25 10.6 7.3 11 80/12 45.2 21.0 20.5 16.1 7.4 11 150/2.25 55.6 28.2 20.5 16.0

This example demonstrates the effect of drying temperature and pH on theinternal crosslinking and also the surface of the CMC. Example 7.2 isinternally crosslinked at high drying temperatures of 150° C. and has alow retention value. When the drying temperature is lowered (Example7.1), there is no internal crosslinking.

Internal crosslinking at high temperatures can be prevented by raisingthe pH, as Example 7.4 shows. The comparison of Example 7.3 with Example7.4 demonstrates the effect of a high temperature on the properties ofthe absorbent resins according to the invention. It is believed thatdrying at high temperatures gives rise to surface hornification, leadingto comparable absorbencies against pressure at a significantly higherretention level.

To control internal crosslinking, the pH of the swelling medium for thepretreatment was herein below adapted in such a way (unless otherwisestated) that drying temperatures of 150° C. provided a precursorretention value of at least 40 g/g which did not decrease to below 40g/g even when the dry precursor was annealed at 120° C. for 60 minutes.

Example 8

Demineralized water is charged to a make-up vessel and mixed withdifferent amounts of 2-propanol. The pH of the solvent is adjusted to apH 11.7 with 4.7 g of 10% strength NaOH per 1000 ml andcarboxymethylcellulose (Cekol® 100,000, degree of neutralization 98.6%)is added with vigorous stirring so that the final concentration of thecarboxymethylcellulose based on the entire batch is between 8 and 20% byweight. After a swell time of 1 hour, the swollen hydrogel was fed to ameat mincer equipped with a mincer plate and comminuted. The comminutedhydrogel was dried in a circulating air cabinet at 150° C. for 2.5hours. The dried hydrogel was coarsely comminuted and ground with aRetsch mill. After the particle size fraction of 150 to 850 μm had beenscreened off, 50 g of each pulverulent precursor were placed in aplastic dish, stirred using a mixer and sprayed with 4.0 g of a 50%strength aluminum sulphate×14H₂O solution in DM water, corresponding to0.36% of Al³⁺ based on CMC, in the course of 10 seconds. The coatedpowder was stirred for a further 110 seconds and subsequently dried at150° C. for 10 minutes. The characteristic absorption data of the driedsurface-crosslinked superabsorbent polymers were then determined asshown in Table 6.

TABLE 6 Example 8 Variations 2-Propanol in CMC in Bulk solvent batch TBAAP_(0.3) AAP_(0.7) density No [wt %] [wt %] [g/g] [g/g] [g/g] [g/dm³]8.1 19.8  8 28.2 20.5 16.0 450 8.2 13.8 12 23.2 17.5 13.5 490 8.3 7.1 1627.3 16.7 13.2 550 8.4 5 20 28.5 16.4 13.4 570 8.5 3 20 28.8 16.0 13.2590 8.6 1 20 28.8 16.0 12.8 610 8.7 0 20 27.7 14.4 12.5 650

Example 8 shows that as little as less than 5% of isopropanol in theswelling medium will reduce the bulk density of the superabsorbentpolymers according to the invention and distinctly improve absorbencyagainst pressure. The results suggest that the rapid vaporization of thesolvent in the course of the drying of the precursors results in anincreasingly porous particle structure, which is beneficial for theabsorbency against pressure in particular.

Example 9

4,800 g of DM water are charged to a make-up vessel and admixed withsodium hydroxide (every 1000 ml of water contain 4.7 g of 10% strengthaqueous sodium hydroxide solution) until the pH is 12. 1,200 g ofcarboxymethylcellulose (Cekol® 100,000, degree of neutralization 98.6%,NaCl content 0.74% by weight) are added with stirring to form a firmhydrogel. After a swell time of 2 hours, the hydrogel was transferredinto a mincer equipped with a mincer plate and comminuted. Thecomminuted hydrogel was dried at 150° C. for 2 hours, coarselycomminuted and ground with a Retsch mill. The particle size fraction of150 to 850 μm was screened off and the characteristic precursor datawere determined:

Example No. 9.1

TB = 54.8 g/g AAP_(0.3) = 8.6 g/g AAP_(0.7) = 8.3 g/g50 g of the screened-off precursor product No 9.1 were initiallycharged, coated with 6 g of a 50% strength solution of Al₂(SO₄)₃×14H₂Oin DM water (0.54% of Al³⁺ based on CMC) with stirring and dried at 150°C. for 10 minutes:

Example No. 9.2

TB = 27.7 g/g AAP_(0.3) = 14.4 g/g AAP_(0.7) = 12.5 g/g50 g of the screened-off precursor product No. 9.1 were initiallycharged, coated with 8 g of a solution of 16.67 g of citric acidmonohydrate and 8.33 g of sodium hypophosphite in 25 g of a 37.5%strength solution of polyethylene glycol 1,500 in DM water (5.1% ofcitric acid, 2.6% of sodium hypophosphite, 3% of PEG based on CMC) withstirring and post crosslinked at 150° C. for 20 minutes.

Example No. 9.3

TB = 23.0 g/g AAP_(0.3) = 14.3 g/g AAP_(0.7) = 11.6 g/g50 g of the screened-off precursor product No. 9.1 were initiallycharged, coated with 7 g of a solution composed of 67% by weight of a50% strength Al₂(SO₄)₃×14H₂O solution in DM water and 33% by weight of a40% citric acid monohydrate solution in DM water (0.43% of Al³⁺ and 1.7%of citric acid based on CMC) with stirring and post crosslinked at 140°C. for 20 minutes:

Example No. 9.4

TB = 26.3 g/g AAP_(0.3) = 14.3 g/g AAP_(0.7) = 12.1 g/g

Example 10

0.5 g of each of the preswollen and crosslinked carboxymethylcelluloseNo. 9.1 and the differently surface-crosslinked superabsorbent polymersNos. 9.2 and 9.3 were transferred into 100 ml of 0.9% strength NaClsolution and stirred at room temperature for 16 hours. Extractables werethen determined by GPC chromatography. The untreated raw material has asolubility of greater than 80% in this analysis.

Example No 9.1 9.2 9.3 Extractables (%) 42 30 21

The pretreatment according to the invention is enough to bring about adistinctly reduced solubility for the precursor compared with the rawmaterial used. The subsequent surface crosslinking further reduces thesolubility.

Example 11

A: A pulverulent carboxymethylcellulose (Cekol® 100,000, degree ofneutralization 98.6%) was coextruded from a twin-screw extruder togetherwith water at different pH values. The total throughput was 56 kg/hourand the carboxymethylcellulose fraction in the hydrogel was 20-25% byweight. The pH of the aqueous solvent was regulated by addition ofsodium hydroxide. The extruder screws were equipped with additionalkneading elements to improve the homogeneity of the hydrogel. Thehydrogel formed was pressed through a breaker plate, the gel extrudatesobtained were dried at 150° C. and subsequently ground and screened offto 150-580 μm. The screened pulverulent precursor product was analyzedfor its retention:

TABLE 7a Example 11 Variations pH of swelling TB No medium [g/g] 11.1 742.9 11.2 8 41.8 11.3 9 41.0 11.4 10 41.1B: A was repeated except that no kneading elements were used at a totalthroughput of 99 kg of hydrogel per hour. The homogeneity of the gel wasvisibly inferior to in A. Here and there the gel extrudates containeddry particles.

TABLE 7b Example 11 Variations pH of swelling TB No medium [g/g] 11.5 743.1 11.6 8 45.2 11.7 9 42.7 11.8 10 41.6C: B was repeated except that the total throughput was 97-102 kg ofhydrogel per hour and the carboxymethylcellulose fraction in thehydrogel was varied between 20 and 45% by weight. The pH of the swellingmedium was adjusted to 7.5 with sodium hydroxide in all cases:

TABLE 7c Example 11 Variations CMC fraction in TB No hydrogel [wt %][g/g] 11.9  23 44.5 11.10 29 41.7 11.11 35 43.1 11.12 40 45.2 11.13 4542.4

50 g of each of the pulverulent precursor products of A, B and C werecharged to a mixing reactor. The surface of the precursor products wascoated with in each case 5 g of a 50% strength aluminum sulphate14-hydrate solution with stirring. The coated products were each driedat 120° C. for 20 minutes and analyzed for their retention values andtheir absorbency against pressure:

TABLE 7d Example 11 Variations Precursor TB AAP_(0.3) AAP_(0.7) No No[g/g] [g/g] [g/g] 11.14 11.1 25.4 16.0 12.6 11.15 11.4 30.2 15.1 11.911.16 11.5 27.0 15.9 12.7 11.17 11.6 31.4 15.9 12.6 11.18 11.7 29.4 15.311.6 11.19  11.11 34.4 15.9 12.9

The results show that the superabsorbent polymers according to theinvention are obtainable on a large scale by continuous processes, theextrusion operation evidently not being subject to any limitations. Evencomparatively less homogeneous gel extrudates, obtained for example byextrusion at high throughput and low water content without the use ofkneading elements, do not lead to any impairment in the absorptionproperties of the absorbent resins according to the invention. Example11, furthermore, demonstrates the conjoint influence of the pH and ofthe mixing technology on internal crosslinking. If, as in this case, themixing operation used continuously supplies the dry raw material withthe swelling medium and the swelling medium is not present in excess atany time, precursors which do not internally crosslink in the course ofdrying are obtained even at pH 7. It is believed that there is arelationship between the degree of solubilization of the raw materialand the reactivity with regard to internal crosslinking. The precedingInventive Examples 1 to 10 had the swelling medium introduced as aninitial charge, i.e. the polysaccharides were present in greaterdilution at the start of the swelling and were able to assume a morereactive preferred conformation. Raising the pH neutralized the freeacid functionalities and hence controlled internal crosslinking.

Example 12

1,200 g of carboxymethylcellulose (Finnfix® 50,000) were preswollen,dried, ground and screened to 150-850 μm, each step being carried out asdescribed in Example 9). 50 g of each screened precursor was coated witha 50% strength solution of Al₂(SO₄)₃×14H₂O in DM water with stirring anddried at 120° C. for 20 minutes. The coated superabsorbent polymers wereanalyzed for their surface crosslinking index:

TABLE 8 Example 12 Variations g of 50% Al₂(SO₄)₃ × 14H₂O sln C_(SAP)C_(F) No 50 g of precursor [Al³⁺]* [Al³⁺]* SCI 12.1 4.00 0.36 0.78 4212.2 5.00 0.45 0.90 45 12.3 7.00 0.64 1.20 56 [ ]*in % by weight

Example 13

The absorbency against pressure performance of the pulverulent absorbentresin Example No. 11.17 (Example 11) was tested using thefluff-absorbent combination test against a synthetic superabsorbent(Z1030 product, synthetic pre- and post crosslinked polyacrylic acidpolymer having a degree of neutralization=70%, AAP_(0.3)=31.6 g/g andAAP_(0.7)=24.4 g/g, from Stockhausen GmbH & Co. KG). The followingcharacteristic data were determined from the absorption curve as afunction of the concentration of superabsorbent (SAP) as shown in Table9:

TABLE 9 FACT at 0.3 psi FACT at 0.7 psi SAP conc. in pad SAP conc. inpad No. Parameter 10% 31% 50% 10% 31% 50% 11.17 Abs_(max) [g/g] 20.029.7 42.3 15.0 23.0 29.0 t_(max) [min] 16 27 84 16 25 47 t_(50%) [min] 13 9 1 2 7 Z1030 Abs_(max) [g/g] 21.8 39.7 68.5 17.4 32.8 53.5 t_(max)[min] 20 29 48 19 36 69 t_(50%) [min] 2 4 9 2 6 12

Example 13 shows that the absorbency of the absorbent resins accordingto the invention can be significantly proved by combination with amatrix material relative to a synthetic polyacrylate absorbent. WhereasExample No. 11.17 has less than 50% of the absorbency against a (0.3 and0.7 psi) pressure value of a Z1030 product, this percentage increases to≧55% in the case of 50% of SAP in the fluff-SAP mix and up to ≧86% inthe case of 10% SAP in the fluff-SAP mix.

Example 14

The crosslinked pulverulent absorbent resin Example No. 11.15 of Example11 was processed with different cellulose fluff quantities to form anairlaid composite article. A synthetic polyacrylate superabsorbent (Z1030, Stockhausen) was processed into a composite article underidentical conditions for comparison. The composites were characterizedfor their retention performance and their liquid absorbency against apressure of 20 or 50 g/cm² as shown in Table 10:

TABLE 10 Airlaid composites Retention LAUL20 (0.3 psi) LAUL50 (0.7 psi)Fluff content SAP Absolute Absolute Absolute in composite (type) [g/m²]Relative* [g/m²] Relative* [g/m²] Relative* 50% 11.15 6 821 86.6 7 78465.0 6 337 67.0 Z 1030 7 878 — 11 963  — 9 452 — 70% 11.15 3 566 82.1 5284 68.9 4 314 72.2 Z 1030 4 345 — 7 673 — 5 974 — 90% 11.15 1 219 86.73 210 81.7 2 568 80.2 Z 1030 1 406 — 3 928 — 3 199 — Characteristic dataof pure pulverulent superabsorbent resins Retention AAP_(0.3) AAP_(0.7)SAP Absolute Absolute Absolute without fluff (type) [g/g] Relative*[g/g] Relative* [g/g] Relative* — 11.15 30.2 97.4 15.1 47.8 11.9 48.7 Z1030 31.0 — 31.6 — 24.4 — *Z 1030 product = 100

Example 14 shows the performance improvement of superabsorbent polymersaccording to the invention relative to synthetic supersuperabsorbentpolymers in airlaid composites. The more homogeneous mixture of thematrix material with the absorbent is responsible for the fact that thisrelative performance improvement is even clearer than in Example 13especially at high absorbent contents in the matrix.

Example 15

Ageing stability was characterized by storing the biodegradablesuperabsorbent resins for a prolonged period at room temperature and anaverage humidity of more than 50% and then measuring retention andabsorbency against pressure.

TABLE 11 Data after synthesis Data after storage Example TB AAP_(0.3)AAP_(0.7) Age in TB AAP_(0.3) AAP_(0.7) No [g/g] [g/g] [g/g] days [g/g][g/g] [g/g] 2.3 19.4 20.9 16.8 513 19.2 19.4 15.4 2.4 20.4 21.8 17.2 38420.5 22.0 15.8 8.1 28.2 20.5 16.0 222 32.6 19.0 14.1 9.2 27.7 14.4 12.5225 25.5 14.1 11.4

Example 16

The particle size distribution of the pulverulent superabsorbentpolymers was determined before and after ball milling and tocharacterize the mechanical stability. The data in the table, whichfollows, are based on the % by weight content of the individual particlesize fractions as shown in Table 12:

TABLE 12 Particle size fraction Particle size fraction before ballmilling after ball milling No 150-300 μm 300-600 μm 600-850 μm 150-300μm 300-600 μm 600-850 μm <150 μm 8.1 12.8 53.6 33.6 16.0 55.6 27.4 1.011.15 19.6 53.7 26.7 21.6 51.9 25.7 0.8 11.17 23.3 52.2 24.5 23.5 52.622.5 1.3 11.19 11.0 61.6 27.4 24.6 50.7 23.4 1.3

This example demonstrates that the superabsorbent polymers according tothe invention are mechanically very robust and will have a similarparticle size distribution and only very small <150 μm fines fractionseven after a mechanical stress of the kind occurring in productconveying for example. This ensures consistent product properties evenafter conveying and metering operations.

The examples show that the polymers according to the invention combinevery high retention ability with a significantly improved ability toabsorb water and aqueous fluids against an external pressure. Theyfurther combine good long-term storage stability with goodbiodegradability under composting conditions. It has also been shownthat only the process according to the invention, involving thepreparation of a hydrogel followed by drying under conditions leading tohornification but not to internal crosslinking and subsequent surfacecrosslinking in minimal layer thickness, will provide the uniquecombination of high retention ability, high absorbency against anexternal pressure, stability in storage and biodegradability.

1. A post crosslinked superabsorbent polymer comprising at least onepartially neutralized, uncrosslinked, carboxyl-containing polysaccharidethat is preswelled and subsequently dried, wherein the driedpolycarboxypolysaccharide is surface-post crosslinked by means of asurface crosslinker wherein said post crosslinked superabsorbent polymerhas an absorbency against pressure (AAP_(0.7)) value of 12.5 g/g ormore.
 2. The superabsorbent polymer of claim 1 wherein thepolycarboxypolysaccharide is derived from starch or cellulose orpolygalactomannan or from any combination of at least two thereof. 3.The superabsorbent polymer of claim 1 wherein polycarboxypolysaccharideincludes carboxyl groups wherein at least 80% of the carboxyl groups areneutralized.
 4. The superabsorbent polymer of claim 1 wherein thecarboxyl groups are attached to the polycarboxypolysaccharide at leastpartially in the form of carboxyalkyl groups.
 5. The superabsorbentpolymer of claim 1 wherein the polycarboxypolysaccharide has an averagedegree of carboxyl group substitution of about 0.3 to about 1.5.
 6. Thesuperabsorbent polymer of claim 1 wherein the uncrosslinkedpolycarboxypolysaccharide is characterized by a solution viscosity for a1% solution of about 2,000 mPas or more.
 7. The superabsorbent polymerof claim 1 further comprising carboxyl-free polysaccharides.
 8. Thesuperabsorbent polymer of claim 1 wherein the surface crosslinker ispresent in an amount of about 0.01 to about 25% by weight, based on thepolycarboxypolysaccharide.
 9. The superabsorbent polymer of claim 1wherein the surface crosslinker is formed by salts of aluminum cationswhich are used in an amount of about 0.2 to about 1.0% by weight, basedon the polycarboxypolysaccharide.
 10. The superabsorbent polymer ofclaim 1 wherein the surface crosslinker is formed by citric acid used inan amount of about 0.2 to about 8% by weight, based on thepolycarboxypolysaccharide.
 11. The superabsorbent polymer of claim 1wherein the surface crosslinker is used in the presence of one or morewater-soluble hydrophilic polymers.
 12. The superabsorbent polymer ofclaim 11 wherein the hydrophilic polymers may be polyalkyleneglycols orpolyvinyl alcohols.
 13. The superabsorbent polymer of claim 1 having aretention of about 15 g/g or more and an absorbency against pressure(AAP_(0.7)) value of 13 g/g or more.
 14. The superabsorbent polymer ofclaim 1 having a retention of about 20 g/g or more and an absorbencyagainst pressure (AAP_(0.7)) value of 14 g/g or more.
 15. Thesuperabsorbent polymer of claim 1 having absorbency against pressure(AAP_(0.7)) value for the polymer powder is not less than about 80% ofthe initial value after ageing for 200 days under standard conditions.16. The superabsorbent polymer of claim 1 having about 5% or less byweight of fines having a particle size of below about 150 microns aftermechanical exposure due to roller milling for 6 minutes.
 17. Thesuperabsorbent polymer powder according to claim 1 having a surfacecrosslinking index (SCD) of about 40 or greater.
 18. A post crosslinkedsuperabsorbent polymer comprising at least one partially neutralized,uncrosslinked, carboxyl-containing polysaccharide that is preswelled anddried and surface-post crosslinking the dried polycarboxypolysaccharideby means of a surface crosslinker, wherein the polycarboxypolysaccharideincludes one or more water-soluble additives from the group consistingof base, salts and blowing agents or one or more anti-blocking additivesfrom the group consisting of natural fiber materials, synthetic fibermaterials, silica gels, synthetic silicas and water-insoluble mineralsalts or any combination of at least two there from and wherein saidpost crosslinked superabsorbent polymer has an absorbency againstpressure (AAP_(0.7)) value of 12.5 g/g or more.
 19. The superabsorbentpolymer of claim 18 wherein the surface crosslinker is formed by saltsof aluminum cations which are used in an amount of about 0.2 to about1.0% by weight, based on the polycarboxypolysaccharide.
 20. Thesuperabsorbent polymer of claim 18 wherein the blowing agents release agas under the influence of a catalyst or heat.
 21. The superabsorbentpolymer of claim 18 wherein the water-soluble additives and antiblockingadditives are each included in amounts of about 0.01 to about 20% byweight, based on polycarboxypolysaccharide.
 22. The superabsorbentpolymer of claim 18 wherein the surface crosslinker is formed by citricacid used in an amount of about 0.2 to about 8% by weight, based on thepolycarboxypolysaccharide.
 23. A post crosslinked superabsorbent polymercomprising at least one partially neutralized, uncrosslinked,carboxyl-containing polysaccharide that is preswelled and subsequentlydried, wherein the polycarboxypolysaccharide is surface crosslinked bymeans of a surface crosslinker, wherein the surface crosslinker may beionic or covalent crosslinkers or any combination of these postcrosslinking.
 24. The superabsorbent polymer of claim 23 wherein theionic surface crosslinkers are salts of at least divalent cations andthe covalent surface crosslinkers are acid-functional substances. 25.The superabsorbent polymer of claim 23 wherein the surface crosslinkeris formed by salts of aluminum cations which are used in an amount ofabout 0.2 to about 1.0% by weight, based on thepolycarboxypolysaccharide.
 26. The superabsorbent polymer of claim 23wherein the surface crosslinker is formed by citric acid of about 0.2 toabout 8% by weight, based on the polycarboxypolysaccharide.
 27. Thesuperabsorbent polymer of claim 23 wherein the covalent surface postcrosslinker is used in the presence of one or more crosslinkingcatalysts.
 28. The superabsorbent polymer of claim 23 wherein the ionicsurface crosslinkers are salts of at least divalent cations and thecovalent surface crosslinkers are acid-functional substances and whereinthe covalent surface post crosslinker is used in the presence of one ormore crosslinking catalysts.
 29. The superabsorbent polymer of claim 23wherein the covalent surface post crosslinker is used in the presence ofone or more crosslinking catalysts selected from the group consisting ofmineral acids, Lewis acids, acetylacetonates and hypophosphites.
 30. Thesuperabsorbent polymer of claim 23 wherein the ionic surfacecrosslinkers are salts of at least divalent cations and the covalentsurface crosslinkers are acid-functional substances and wherein thecovalent surface post crosslinker is used in the presence of one or morecrosslinking catalysts selected from the group consisting of mineralacids, Lewis acids, acetylacetonates and hypophosphites.
 31. Anabsorbent post crosslinked polymer comprising at least one partiallyneutralized, uncrosslinked, carboxyl-containing polysaccharide that ispreswelled and subsequently dried and a post crosslinking surfacecrosslinker, wherein the surface post crosslinking utilizes ionic orcovalent crosslinkers or a combination thereof, wherein the ionicsurface crosslinkers are salts of at least divalent cations and thecovalent surface crosslinkers are acid-functional substances, whereinthe polyvalent cation is formed from the group consisting of Mg²⁺, Ca²⁺,Al³⁺, Ti⁴⁺, Fe³⁺,Fe²⁺, Zn²⁺, and Zr⁴⁺, and the acid-functionalsubstances are formed from low molecular weight and polymericpolycarboxylic acids and wherein the absorbent post crosslinked polymerhas an absorbency against pressure (AAP_(0.7))value of 16 g/g or more.32. The superabsorbent polymer of claim 31 wherein the surfacecrosslinker is present in an amount of about 0.01 to about 25% byweight, based on the polycarboxypolysaccharide.
 33. The superabsorbentpolymer of claim 31 wherein the surface crosslinker is formed by saltsof aluminum cations which are used in an amount of about 0.2 to about1.0% by weight, based on the polycarboxypolysaccharide.
 34. Thesuperabsorbent polymer of claim 31 wherein the surface crosslinker isformed by citric acid in the amount of about 0.2 to about 8% by weight,based on the polycarboxypolysaccharide.
 35. The superabsorbent polymerof claim 31 wherein the covalent surface post crosslinker is used in thepresence of one or more crosslinking catalysts.
 36. The superabsorbentpolymer of claim 31 wherein the covalent surface post crosslinker isused in the presence of one or more crosslinking catalysts selected fromthe group consisting of mineral acids, Lewis acids, acetylacetonates andhypophosphites.
 37. The superabsorbent polymer of claim 35 wherein theratio by weight of the surface post crosslinker to the crosslinkingcatalysts is from about 1:0.001 to about 1:1.
 38. An absorbent postcrosslinked polymer from a process comprising the steps of a) forming ahydrogel by mixing an uncrosslinked polycarboxypolysaccharide that ispreswelled and subsequently dried with water; b) the hydrogel ismechanically comminuted and dried; c) the dried hydrogel is comminutedand classified to form a polymer particles; and d) the polymer particlesare coated with a solution of a crosslinker and subjected to a surfacepost crosslinking and wherein the absorbent post crosslinked polymer hasan absorbency against pressure (AAP_(0.7)) value of 12.5 g/g/ or more.39. Absorbent hygiene product comprising an superabsorbent polymercomprising at least one partially neutralized, uncrosslinked,carboxyl-containing polysaccharide that is preswelled and subsequentlydried, wherein the dried polycarboxypolysaccharide is surface-postcrosslinked by means of a surface crosslinker.
 40. The absorbent hygieneproduct of claim 39 wherein the polycarboxypolysaccharide is derivedfrom starch or cellulose or polygalactomannan or from any combination ofat least two thereof.
 41. The absorbent hygiene product of claim 39wherein polycarboxypolysaccharide includes carboxyl groups wherein atleast 80% of the carboxyl groups are neutralized.
 42. The absorbenthygiene product of claim 39 wherein the carboxyl groups are attached tothe polycarboxypolysaccharide at least partially in the form ofcarboxyalkyl groups.
 43. The absorbent hygiene product of claim 39wherein the polycarboxypolysaccharide has an average degree of carboxylgroup substitution of about 0.3 to about 1.5.
 44. The absorbent hygieneproduct of claim 39 wherein the uncrosslinked polycarboxypolysaccharideis characterized by a solution viscosity for a 1% solution of about2,000 mPas or more.
 45. The absorbent hygiene product of claim 39further comprising carboxyl-free polysaccharides.
 46. The absorbenthygiene product of claim 39 wherein the surface crosslinker is presentin an amount of about 0.01 to about 25% by weight, based on thepolycarboxypolysaccharide.
 47. The absorbent hygiene product of claim 39wherein the surface crosslinker is formed by salts of aluminum cationswhich are used in an amount of about 0.2 to about 1.0by weight, based onthe polycarboxypolysaccharide.
 48. The absorbent hygiene product ofclaim 39 wherein the surface crosslinker is formed by citric acid usedin an amount of about 0.2 to about 8% by weight, based on thepolycarboxypolysaccharide.
 49. The absorbent hygiene product of claim 39wherein the surface crosslinker is used in the presence of one or morewater-soluble hydrophilic polymers.
 50. The absorbent hygiene product ofclaim 39 wherein the superabsorbent polymer has a retention of about 15g/g or more and an absorbency against pressure (AAP_(0.7)) value of 15g/g or more.
 51. The absorbent hygiene product of claim 39 wherein thesuperabsorbent polymer has absorbency against pressure (AAP_(0.7)) valuefor the polymer powder is not less than about 80% of the initial valueafter ageing for 200 days under standard conditions.
 52. The absorbenthygiene product of claim 39 wherein the superabsorbent polymer has about5% or less by weight of fines having a particle size of below about 150.mu.m after mechanical exposure due to roller milling for 6 minutes. 53.The absorbent hygiene product of claim 39 wherein the superabsorbentpolymer powder has a surface crosslinking index (SCI) of about 40 orgreater.
 54. Absorbent hygiene product comprising an superabsorbentpolymer comprising at least one partially neutralized, uncrosslinked,carboxyl-containing polysaccharide that is preswelled and dried andsurface-post crosslinking the dried polycarboxypolysaccharide by meansof a surface crosslinker, wherein the polycarboxypolysaccharide includesone or more water-soluble additives from the group consisting of bases,salts and blowing agents or one or more anti-blocking additives from thegroup consisting of natural fiber materials, synthetic fiber materials,silica gels, synthetic silicas and water-insoluble mineral salts or anycombination of at least two there from and wherein the absorbent postcrosslinked polymer has an absorbency against pressure (AAP_(0.7)) valueof 12.5 g/g or more.